1. Field of the Invention
This invention relates generally to the field of thermal treatment of substrate and, more particularly, to the batch-wise thermal treatment of substrates wherein a plurality of substrates are accommodated in a vertically spaced-apart relationship in a substrate holder during the thermal treatment in a furnace.
2. Description of the Related Art
The batch-wise thermal treatment of substrates, such as semiconductor wafers, in vertical furnace is known in the art. In a vertical furnace, substrates are accommodated in a vertically spaced-apart relationship in a substrate holder (e.g., vertical wafer boat) during the thermal treatment. Typically, the substrates are supported at their edge in such a substrate holder. With the increasing diameter of semiconductor wafers, while their thickness was only marginally increased, these wafers became more and more susceptible to plastic deformation during thermal treatment. Unfortunately, the mechanical strength of the substrate reduces at high temperature and the stresses in the substrate, due to gravitational forces and temperature gradients, can easily exceed the yield strength of the substrate at temperatures at 900xc2x0 C. or above.
As a solution, substrate holders were proposed that support the substrates not only at the edge but also at an inner area, spaced from the edge. In this way, gravitational stresses are strongly reduced. Various designs for such substrate holders were proposed, as exemplified by, U.S. Pat. No. 5,931,666, to Hengst and U.S. Pat. No. 5,492,229 to Tanaka. These substrate holders are effective in preventing plastic deformation of the substrates. However, another problem became apparent: scratches were observed on the front side of the substrates. The positions of the scratches on the substrates correspond with the positions of the support members of the substrate holders.
It is an object of the present invention to provide a solution for the occurrence of scratches on the front sides of substrates when they are subjected to a thermal treatment while still supporting the substrate at an inner area.
In accordance with one aspect of the invention, a substrate holder is configured to support a substrate at an inner area. The holder allows sufficient clearance above substantially the entire front face of the substrate that a substrate deflection or bow, induced by thermal stresses during loading and unloading of the substrate holder into and out of the furnace, can be accommodated without the substrate touching the support members of the substrate holder. Sufficiently low loading and unloading temperatures are selected in conjunction with the amount of free space in the substrate holder accommodation or slot.
In accordance with another aspect of the invention, a substrate holder is configured to hold a plurality of substrates in a vertically spaced-apart relationship. The substrate holder includes a plurality of support accommodations that at least partially support a substrate at an inner area spaced from the substrate edge. Each support accommodation has a free height between vertically spaced support members of between 4.5 mm and 12 mm.
In accordance with another aspect of the invention, a method is provided for thermal treatment of substrates. The method includes providing a holder with a plurality of accommodations in a vertically spaced-apart relationship. Each accommodation is capable of supporting a substrate horizontally at an inner area, which is spaced from the substrate edge by at least 20 mm, and each accommodation has a free height (h). A plurality of substrates is supported in the holder. The substrate holder with the supported substrates is loaded into a vertical furnace at a loading temperature (Tload). The furnace is then heated to a thermal treatment temperature and the thermal treatment is performed. After performing the thermal treatment, the furnace is cooled to an unloading temperature Tunload. The substrate holder is unloaded from the furnace. The method also includes selecting one of (h) and (Tunload) and determining the other of (h) and (Tunload) according to a predetermined relationship between the free height and the unloading temperature in which scratching of the substrates is avoided.
In accordance with another aspect of the invention, a method is provided for avoiding scratches on substrates supported in a vertical furnace, in which each substrate is at least partially supported at a point at least 20 mm from the substrate edge. The method includes providing a substrate rack with a plurality of vertically stacked support slots, where the slots have a set free height between the lower surface of a first supported substrate and a support structure above the supported substrate. An unloading temperature is selected to avoid substrate scratching for the set free height. A batch of substrates supported in the substrate rack are processed at a process temperature. The substrates are then cooled from the process temperature to the selected unloading temperature, and the rack with the substrates is unloaded at the selected unloading temperature.